Life must self-replicate, a task that is mostly accomplished with DNA today. Some scientists think that the first replicator was not DNA but RNA. They call this the RNA world hypothesis.

Metabolism

Organic molecules must also carry out the chemical work of cells; that is, their metabolism. Chemical reactions in a living organism allow that organism to live in its environment, grow, and reproduce. Metabolism gets energy from other sources and creates structures needed in cells. The chemical reactions occur in a sequence of steps known as metabolic pathways. The metabolic pathways are very similar between unicellular bacteria that have been around for billions of years and the most complex life forms on Earth today. This means that they evolved very early in Earth's history.

Replication

Living cells need organic molecules, known as nucleic acids, to store genetic information and pass it to the next generation. Deoxyribonucleic acid (DNA) is the nucleic acid that carries information for nearly all living cells today and did for most of Earth's history. Ribonucleic acid (RNA) delivers genetic instructions to the location in a cell where protein is synthesized.

RNA World

Many scientists think that RNA was the first replicator. Since RNA catalyzes protein synthesis, most scientists think that RNA came before proteins. RNA can also encode genetic instructions and carry it to daughter cells, such as DNA.

The idea that RNA is the most primitive organic molecule is called the RNA world hypothesis, referring to the possibility that the RNA is more ancient than DNA. RNA can pass along genetic instructions as DNA can, and some RNA can carry out chemical reactions like proteins can.

Pieces of many scenarios can be put together to come up with a plausible suggestion for how life began.

Summary

An organism's metabolism is the chemical reactions that allow it to live, grow, and reproduce.

Nucleic acids pass genetic information to the next generation: DNA for living cells, and RNA for protein synthesis.

The RNA world hypothesis suggests that RNA was the first nucleic acid to evolve and DNA came later.

Review

What is the purpose of an organism's metabolism?

Why is the fact that metabolic pathways are similar between organisms significant?

Explain the RNA world hypothesis.

Explore More

Use this resource (watch up to 6:23) to answer the questions that follow.

What does DNA do? What does that mean?

How many chromosomes does every cell in a human have? How many DNA molecules does each cell have? Where are the DNA molecules in human cells?